1. Field of the Invention
This invention relates to a combustion state-detecting system for detecting a state of combustion (misfiring) in internal combustion engines, based on a variation in the angular velocity of the crankshaft during rotation of the engine in every combustion cycle.
2 . Prior Art
To detect a cylinder of an internal combustion engine in which normal combustion does not take place due to failure in the ignition system, the fuel supply system, or the like, an abnormality-detecting device has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 3-286166, which detects a rate of variation in the angular velocity of the crankshaft during rotation of the engine in every combustion cycle, to thereby determine whether or not an abnormality has occurred in any of the cylinders, based on the detected rate of variation in the angular velocity of the crankshaft.
According to this conventional device, an output from a crank angle sensor provided at the crankshaft is directly supplied to the abnormality-detecting device, and the pulse repetition period of the sensor output is measured to detect a rotational speed, i.e. the angular velocity, of the crankshaft. However, the above device suffers from the following inconveniences:
(1) A pulser, which constitutes the crank angle sensor, requires as high an accuracy as is almost equal to a limit value obtainable by mechanical working, and therefore, dimensional errors produced during manufacturing of the device can result in an erroneously large rate of variation in the rotational speed of the crankshaft, based on which a normal combustion can be erroneously determined to be abnormal, i.e. a misfire. PA1 (2) The rate of variation in the rotational speed of the crankshaft also varies with different output characteristics among individual engines, which can lead to an erroneous determination of a misfire, similarly to the above. PA1 (3) When a vehicle on which the engine is installed is travelling on a bad road or the like, a large variation in the rotational speed of the crankshaft occurs due to vibrations of wheels of the vehicle, which can also cause the device to erroneously determine that a misfire has occurred. PA1 first averaging means for calculating a first average speed by averaging the rotational speed of the engine over a time period corresponding to duration of one stroke of one of the at least one cylinder; PA1 second averaging means for calculating a second average speed by averaging the rotational speed of the engine over a time period corresponding to duration of one cycle of all strokes of the one of the at least one cylinder; and PA1 combustion state-detecting means for calculating a difference between the first average speed and the second average speed, and determining based on the difference whether or not the combustion state of the engine is abnormal.